Senators negotiating a bill mandating background checks for all gun buyers are privately expecting the National Rifle Association not to fight the measure -- provided the legislation does not require private gun sellers to maintain records of the checks, NBC News has learned.

If that requirement is met and key Republican negotiator Sen. Tom Coburn of Oklahoma signs on, the powerful gun lobby has signaled to lawmakers that they would not actively oppose the bill -- and not count votes in favor of it as part of its highly influential NRA lawmaker ratings -- according to Senate aides familiar with the stalled negotiations.

Such a deal could clear the way for a universal background check bill, a central tenet of President Barack Obama's gun control initiative, to pass the Senate with significant Republican support. Odds of passage in the House would brighten significantly as a result.

The NRA denies being part of any agreement. "We do not take positions on hypotheticals. We will make our position known if and when legislation is introduced," said Chris Cox, the group's top lobbyist.

The NRA is still adamantly opposed to expanding background checks to private sales. "To be clear, the National Rifle Association does not support legislation that would criminalize otherwise lawful transfers of firearms between law abiding Americans," Cox said.

Such a decision would mark the first major compromise from the group in the wake of the elementary school shootings that killed 26 in Newtown, Conn. The NRA has signed on to a bipartisan mental health bill and are talking to senators about gun trafficking legislation. But with an assault weapons ban not likely to pass, the background check bill is the main legislative fight over new gun restrictions.

Still, Democrats have balked at this development: Leaving such a provision out, they say, would make the law toothless and would never earn support from gun control advocacy groups.

But without the record-keeping provision -- and the blind eye from the NRA -- Democrats would instead have to squeak their way to the 60 votes needed to pass in the upper chamber, potentially risking the whole package.

Both sides say it's tough to compromise on the issue of records. Coburn and the NRA argue that requiring private sellers to keep a record of each background check and gun sale would start down a slippery slope to a national gun registry. Gun control advocates, on the other hand, say enforcing any background check law would be all but impossible if records aren’t kept, and thus would have little impact on the criminals the bill is designed to target.

"With about 90 percent of the public calling for a comprehensive background check system, the incentive to agree to a bill that only law abiding people will comply with, and that people we're trying to reach will ignore, is vanishingly small," said Mark Glaze, the executive director of Mayors Against Illegal Guns, a group headed by New York City Mayor Michael Bloomberg.

"Background checks do not and cannot lead to gun registration. It's a lie meant to muddy the debate and distract from our common goal -- saving lives with solutions that Americans support overwhelmingly," Mark Kelly, the husband of former Rep. Gabby Giffords, said in a Tuesday statement.

Coburn, who holds an "A" rating from the NRA, has been locked in closed meetings with Sens. Chuck Schumer, D-N.Y., and Joe Manchin, D-W.Va., over the background check legislation.

Coburn and Manchin insist they're still talking, and Manchin said he was on the phone Tuesday with the NRA. He and Coburn have shared legislative language with the group as they've worked on an agreement.

"Tom Coburn and I come from a culture, a background, a culture of guns. So I think the NRA would respect and appreciate where we're coming from, with the understanding that we understand our NRA members," Manchin told NBC News on Tuesday. "I'm an NRA member, Tom's an NRA member, and they're looking at that. And hopefully we can find some common ground."

But the background check discussions with Schumer have stalled -- and the New Yorker is now reaching out in an effort to find another Republican who might be willing to sign on. The list includes moderates, like Sen. Susan Collins of Maine, and Republican senators who aren't running for re-election, like Georgia Sen. Saxby Chambliss.

Chip Somodevilla / Getty Images

Sen. Chuck Schumer boards an elevator at the U.S. Capitol on March 12, 2013 in Washington, D.C.

Sources familiar with the negotiations suggest that Schumer has only made a public break with Coburn because he's trying to convince groups on the left -- the Bloomberg group, the Brady Campaign to Prevent Gun Violence, and others -- that they need to give in on record-keeping in order to save a broader package.

Glaze said that's a no-go, indicating that his group -- backed by Bloomberg's billions -- would instead air ads in senators' home states attacking them for not going further in their quest to stop gun violence.

"We're going to take the time to get out into the country and have that conversation with members and their constituents, and we will see where we end up in the late summer," Glaze said.

Asked about the groups' position Tuesday, Manchin emphasized the realities of getting a bill passed.

"I respect their position, but basically, we have 535 members, and they all have certain concerns and considerations to be taken. If you want to pass something, you have to work within a, a compromise if you will, bipartisan arena," he said.